Chiral complexes of titanium containing a linked amido-cyclopentadienyl ligand: synthesis, structure, and asymmetric imine hydrogenation catalysis

A series of mono- and disubstituted derivatives (−)-(S)-Ti(η5:η1-C5Me4SiMe2NCHMePh)(X)Cl (X=CH2SiMe3, BH4) and (−)-(S)-Ti(η5:η1-C5Me4SiMe2NCHMePh)X2 (X=OSO2CF3, OiPr, Me, CH2Ph) was prepared from (−)-(S)-Ti(η5:η1-C5Me4SiMe2NCHMePh)Cl2 without significant racemization at the stereogenic center. The monosubstituted complexes are formed as mixtures of diastereomers. One diastereomeric monoalkyl (STi, SC)-Ti(η5:η1-C5Me4SiMe2NCHMePh)(CH2SiMe3)Cl was characterized by X-ray single crystal structure analysis. When the (−)-(S)-NCHMePh group is attached to planar chiral ring moieties 3-tBuC5H3, C9H6, and C9H5(SiMe3)-3 and coordinated at the titanium center, diastereomeric mixtures are formed. A series of titanium complexes Ti(η5:η1-C5R4SiMe2NRʹ)Cl2 (R=H, Me; R′=CHMeC10H7, CHMeCMe3, CHPhCMe3, CHMeC6H11, (1S)-pinanyl-3, (1R)-bornyl-2) containing an enantiomerically pure linked amido-cyclopentadienyl ligands were synthesized and characterized by 1H- and 13C-NMR spectroscopy, mass spectrometry, and elemental analysis. The crystal structure of a three-legged piano-stool molecule was determined for (+)-(1S)-Ti(η5:η1-C5Me4SiMe2NCH2pinanyl-3)Cl2 by a single-crystal X-ray diffraction study. Upon activation with n-butyllithium a selection of these dichloro complexes catalyzed the hydrogenation of acetophenone N-benzylimine with good conversions for R=H, but with low enantioselectivities.